which movie is the best to a Poliwhirl defend a gym from a grass type attacker ?

Ice Punch is the obvious choice, since your Poliwrath will use it very often. Anyoke attacking against it would always pick something that resist Bubble, and Hydro Pump, being already easy to dodge, won't really be effective...

Ice punch is actually a great choice if what concerns you is only defending.
The DPS you saw is simply DMG/AnimationTime. But when you are defending, the animation time doesn't matter, damage per hit is all that matters. Since Ice Punch has a great DMG/EnergyCost ratio, its actually great even if the attacker isn't vulnerable to it.

The Grass Eater from year 1961 sounds like your movie.
No idea what its all about, but i wish you good movie night.